[{_id:ObjectId("628fb1db596e46baf54fb8fc"),string:"a","list of objects":[{"string_1":"2","string_2":"2","string_3":"1",},{"string_1":"2","string_2":"2","string_3":"4",},{"string_1":"3","string_2":"5","string_3":"3",},]}]

db.collection.aggregate([{$unwind:{path:"$list of objects",includeArrayIndex:"index"}},{$group:{_id:{string_1:"$list of objects.string_1",string_2:"$list of objects.string_2"},string_3:{$first:"$list of objects.string_3"},string:{$first:"$string"},index:{$first:"$index"},origId:{$first:"$_id"}}},{$sort:{index:1}},{$group:{_id:"$origId","list of objects":{$push:{string_1:"$_id.string_1",string_2:"$_id.string_2",string_3:"$string_3"}},string:{$first:"$string"}}}])